1. Field of the Invention
The present invention relates to an image reading apparatus configured to read an image formed on an original, while conveying the original, and a shading correction method therefor as well as a program for implementing the method.
2. Description of the Related Art
In general, in an image reading apparatus configured to read an image formed on an original, while conveying the original, the original is illuminated with light from a light source, and reflected light from the original is focused by a rod lens array or the like, whereby the image on the original is read e.g. by a line image sensor. However, due to variations in the amounts of light from the light source and the variations in the performance of the rod lens array and variations in the sensitivity of the line image sensor, it is impossible to perform uniform image reading without any correction.
To enable uniform image reading, image data obtained from an output signal from the line image sensor when an original image is read is corrected using shading correction data formed based on an output signal from the line image sensor obtained when a white reference member is read. When correction data for use in the correction is captured, light amount adjustment for optimizing the amount of light emitted from the light source to illuminate an original, and gain adjustment for optimizing an amplification factor for amplifying image signal output from the line image sensor are carried out. Further, it is a general practice to perform shading correction for correcting the variations in the amounts of light from the light source and the variations in the performance of the rod lens array and the variations in the sensitivity of the line image sensor in association with each pixel thereof. Hereafter, the correction including the light amount adjustment and the gain adjustment performed so as to enable the line image sensor to uniformly read image information from the original will be referred to as “shading correction”.
In an image reading apparatus of the above-mentioned type, a member disposed in an original image reading position where the line image sensor reads an image on an original, for supporting a conveyed original from the reverse side of the same original has a color (black in general) other than white for the purpose of detection of a boundary between the original image and the background image, detection of a skew-feeding of the original, and prevention of lack of hiding of a reverse side image of the original.
A description will be given of the schematic arrangement and operation of a conventional typical image reading apparatus with reference to FIG. 18.
The image reading apparatus 1000 shown in FIG. 18 is configured to read image information on an original D by a line image sensor 110 through a contact glass 150 while conveying the original D. The image reading apparatus 1000 includes a pickup roller 2 for picking up originals D, a feed roller 3 for feeding the originals D picked up by the pickup roller 2, a retard roller 4 for separating the picked-up originals D one from another, and a registration roller pair 5 and a convey roller pair 7 each formed by a pair of rollers disposed at respective opposed locations, for conveying the originals D.
First, before reading an image on an original D, the image reading apparatus 1000 moves the line image sensor 110 in a direction indicated by an arrow S in FIG. 18 and causes the line image sensor 110 to read a reference member 170. The reference member 170 is disposed at a location opposed to the line image sensor 110 having been moved in the direction indicated by the arrow S.
Then, image data obtained based on an output from the line image sensor 110 when the line image sensor 110 has read the reference member 170 is stored, as shading correction data for shading correction, in association with each pixel of the line image sensor 110. Thereafter, the image reading apparatus 1000 returns the line image sensor 110 to its original position (original image reading position), and causes the line image sensor 110 to read the image on the original D, while conveying the original D. During the operation for reading the original D, the image data obtained from the output of the line image sensor 110 is corrected by shading correction performed with reference to the correction data stored in advance. It should be noted that whether or not the line image sensor 110 has been moved to a reference member reading position for reading the reference member 170 can be determined based on an output from a position sensor 120. The position sensor 120 is a means for detecting the position of the line image sensor 110.
In the image reading apparatus 1000 configured as above, if the line image sensor 110 deviates from the exact original image reading position when the line image sensor 110 returns from the reference member reading position to the original image reading position, registration error occurs in original reading. This registration error is generally prevented by positioning the line image sensor 110 using the position sensor 120. The above-described sequential operation is commonly performed in response to an instruction from a control means, such as a CPU (Control Processing Unit).
Some image reading apparatuses are configured such that not a line image sensor but a reference member is moved between original reading position and escape position. For example, a technique has been proposed in which the reference member is exposed into an original conveying path during a period of reference member reading, and is retracted to a position where the reference member does not contact with a conveyed original, during a period of original reading (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017).
In an image reading apparatus configured as above such that the relative position between the line image sensor and the reference member is switched between the period of reference member reading and the period of original reading, it is a general practice to detect the image reading position of the line image sensor by the position sensor 120. This is because there is a fear of occurrence of not only the above-mentioned registration error but also damage to the reference member or breakage of a very original.
On the other hand, an image reading apparatus has also been proposed which is configured such that the image reading position is detected without using the position sensor 120. Specifically, first, a reference position detection mark is formed on the reference member, and the line image sensor is moved to a position opposed to the reference member to read the image. Then, the position of the reference position detection mark is determined based on the read image data to thereby determine the original image reading position and the reference member reading position (see e.g. Japanese Laid-Open Patent Publications (Kokai) No. H04-196669 and No. 2004-120599).
In the following, a description will be given of a shading correction mechanism of a conventional image reading apparatus (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017).
FIGS. 19A and 19B are schematic cross-sectional views of an image reading section of the conventional image reading apparatus, in which FIG. 19A shows a state of the image reading section during an original reading period, and FIG. 19B shows a state of the same during a reference member reading period.
As shown in FIG. 19A, during the original reading period, a shading sheet member 54 as the white reference member is kept away from a platen roller 51 as a black opposed member, an image sensor 1901, and an original conveying path.
In addition to the shading sheet member 54, the shading correction mechanism includes an arm 56 as a first swinging member which is swingable, and a sheet table 1903 as a second swinging member which swings about a shaft 1902 on the arm 56. The sheet member 54 is deformably attached to the sheet table 1903.
The arm 56 is held in contact with a stopper 53 by being pulled by a resilient member 59, such as a tension spring. This position is a standby position of the shading sheet member 54 during a period of original conveying.
Further, one end of a gear 58 is attached to a plunger 52 of a solenoid 55, and when the solenoid 55 is energized, the plunger 52 is attracted to the solenoid 55, whereby the gear 58 rotates about a shaft 60 in the clockwise direction. The arm 56 in mesh with the gear 58 rotates about a shaft 57 in the counterclockwise direction in a manner interlocked with the clockwise rotation of the gear 58.
On the other hand, when the platen roller 51, the image sensor 1901, and the original conveying path come into contact with the shading sheet member 54, and the counterclockwise rotation of the arm 56 is stopped, the image reading section enters a state capable of reading the reference member, as shown in FIG. 19B.
More specifically, when the solenoid 55 is energized, the shading sheet member 54 is brought into a gap between the platen roller 51 and the image sensor 1901 by actuation via the gear 58, the arm 56, and the sheet table 1903.
After shading correction data is acquired in this state, when the solenoid 55 is deenergized, the shading sheet member 54 is returned to the above-mentioned standby position by the tension of the resilient member 59.
It should be noted that a plate member, such as a guide plate, can be used as an opposed member to the image sensor 1901 in place of the platen roller 51. Further, the image sensor 1901 can be moved for acquisition of shading correction data, instead of moving the shading sheet member 54.
However, in the image reading apparatus disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017, which is capable of reading both sides of an original, reference members and shading correction mechanisms are required to be disposed at respective locations at which image sensors associated therewith are opposed thereto, and hence the degree of freedom of design is reduced, which makes it difficult to reduce the size and weight of the apparatus.
Further, in the conventional image reading apparatus shown in FIG. 18, it is required to provide the position sensor 120 so as to prevent registration error. Furthermore, in the image reading apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-102017, since the shading correction mechanisms are provided for causing the reference members to be exposed into the original conveying path, the number of component parts is increased, which complicates the construction of the apparatus.
In addition, in the conventional image reading apparatus, since it is required to provide the shading correction mechanisms in association with the image sensor for reading the front side of an original and the image sensor for reading the reverse side of the same, respectively, the number of component parts is increased, which further complicates the construction of the apparatus. Further, in the conventional image reading apparatus capable of reading of both sides of an original, the locations where the image sensors and those of the shading correction mechanisms can be disposed are limited, and hence the degree of freedom of design is reduced, which makes it difficult to reduce the size and weight of the apparatus.
Further, the conventional image reading apparatus is comprised of an upper unit and a lower unit arranged such that the upper unit can be opened and closed about a hinge, and therefore the opening/closing of the upper unit during a reading operation can cause failure of a shading correction mechanism or breakage of a component part. Furthermore, the image sensors of the conventional image reading apparatus are fixed to the respective upper and lower units, so that in reading a thick original, there is a fear that the original cannot be smoothly conveyed.
Moreover, when the position sensor is disposed in the vicinity of a traveling path of the line image sensor or on the line image sensor, the movement of the line image sensor can be hindered by the position sensor, which increases mechanical limitation.
Even in a case where the position sensor is disposed at a location away from the line image sensor, if a member that operates in a manner interlocked with movement of the line image sensor is additionally provided, the above-mentioned determination can be performed. However, installation of the linkage member not only restricts the construction of the apparatus, but also hinders reduction of the size and weight of the same.
Further, the provision of the position sensor itself leads to an increase in the entire manufacturing cost of the apparatus.
On the other hand, in a case where a reference position mark is formed on the reference member, the reference position mark cannot be disposed on a line along which acquisition of shading correction data is performed by the line image sensor. For this reason, it is necessary to form the reference position mark at a location other than the line for acquisition of shading correction data by the line image sensor, and hence the size of the reference member having the reference position mark additionally formed thereon is inevitably increased, which hinders reduction of the size of the product, i.e. the image reading apparatus. In addition, the reference position mark is required to be formed on the reference member using a member different in color from the reference member, which leads to an increase in the manufacturing cost of the reference member.
Further, unless installation of the position sensor and formation of the reference position mark on the reference member are performed with accuracy, the positioning undesirably varies from one apparatus to another. For this reason, it is required to adjust the location of the position sensor or the reference member and the distance between the reference member reading position and the original image reading position of the line image sensor on an apparatus-by-apparatus basis, which leads to an increase in manufacturing costs.
Further, as shown in FIGS. 19A and 19B, in the above-described conventional image reading apparatus, the mechanism for moving the reference member is complicated, which causes an increase in manufacturing costs. Furthermore, since the mechanism for moving the reference member is formed as a unit independent of the image sensor, adjustment and positioning of component parts is required during assembly, which brings about the problem of degraded assemblability.
Moreover, in the conventional image reading apparatus, the image sensors for reading respective images on both sides of an original and the respective mechanisms for moving the reference members are provided for both sides of the original, the number of component parts is increased, which leads to an increase in manufacturing costs.